1. Field of the Invention
The invention relates to a method and to a device for monitoring and controlling the temperature of a sample by determining its temperature-dependent refractive index.
Obvious applications of the present invention occur in the field of medical interventions, where e.g. by means of electromagnetic radiation, particularly laser light, temperature rises can be produced and simultaneously monitored and controlled in a biological tissue, e.g. in the retina of the eye.
2. Description of Prior Art
DE 199 35 455 A1 discloses a method and a device for planned heat deposition in a biological material. For this purpose ultrasonic waves are fed into a tissue and detected in time and space-resolved manner at an appropriate location. From a computer-assisted analysis of the emerging waves, particularly the degree of their relative transit time changes, information is obtained on the thermal and structural changes within the material, which are in turn used for controlling the heat quantity introduction, e.g. per laser light.
This method is not very suitable for ophthalmology due to the lack of space resolution with low frequency ultrasound and due to the strong sound absorption with high resolving, high frequency ultrasound and low power. However, there is a risk of damage to the sensitive retinal tissue in the case of strong excitation with mechanical waves.
It is obvious when measuring the temperature in the eye to make use of mainly optical methods, such as is e.g. implemented with known infrared ear thermometers. However, the vitreous body of the eye absorbs infrared light, so that it has hitherto been impossible to simply adapt such a thermometer for ophthalmological purposes.
DE 101 35 944 A1 discloses a device, in which a low power probing laser ensures a brief expansion of the tissue to be treated using regular light pulses. The expansion leads to the transmission of a pressure wave, which runs through the vitreous body and can be externally detected by means of a contact lens. The sensor acts as an ultrasonic receiver and transmits its data to a computer, which in turn controls the energy supply of a power laser.
DE 102 40 109 A1 describes another method, in which the temperature of the fundus oculi is determined by exciting to fluoresce. Changes to the spectral composition, the intensity or decay time of the fluorescent light are linked with a temperature rise compared with the normal level (approximately 37° C.). The fluorescent activity is due to dyes, which are either naturally concentrated with rising age in the eye, such as e.g. lipofuscin, or are introduced into the eye for medical treatment purposes.
An indirect access to the temperature of a sample is provided by the dependence of the refractive index on the sample temperature documented for numerous substances in the literature. Of particular interest for the biological tissue is the refractive index of water, which is described in summary form in the work by Thormahlen, Straub and Grigull, “Refractive Index of Water and its Dependence on Wavelength, Temperature and Density”, J. Phys. Chem. Ref. Data. 14, 933-944 (1985). However, in practical terms the refractive index-based temperature measurement is hardly used, because virtually always simpler and more precise alternatives are available.
U.S. Pat. No. 4,468,136 describes a method for measuring the temperature distribution in the surface-near area of a sample under the action of a locally defined laser beam vertically striking the sample. Use is made of the formation of a thermal lens in the material, i.e. as a result of local temperature gradients there is a space-dependent differentiation of the refractive index and light is then deflected in glancing or surface-parallel incidence. The extent and direction of the deflection are dependent on the position and propagation direction of the “probing” light beam relative to the heating centre by the power laser.
The prerequisite for performing the method is an at least extensive transparency of the material for the probing light, particularly low absorption and low scattering, along the sample surface. In the case of biological samples this can only be achieved with high energy light, so that, apart from apparatus difficulties, there are objections to this method from the medical standpoint.
DE 39 29 290 A1 describes a measuring cell with which inter alia the ambient temperature of the cell is determined via the change to the optical path length for laser light in a medium with temperature-dependent refractive index in the interior of the cell. This change is determined interferometrically according to the known principle of interference on layers, in which the transit time difference between reflected partial beams of two parallel, optionally partly reflecting interfaces (such as the refracting medium) is measured and interpreted.
However, specifically DE 39 29 290 A1 is based on the precise knowledge of the refractive index as a function n(T,p,) by the design-side presetting of the refracting medium and the “good thermal contact” of said medium with the environment.
Non-invasive methods for determining light transit time distributions, particularly of infrared light in the biological tissue, are known as “Optical Coherence Tomography” (OCT). Thus, DE 199 29 406 A1 describes a device which simulates the transit time distribution by means of a structure based on the known double slit experiment in a detection unit in the form of an interferogram. For this purpose, initially short coherence length light is split up into a reference beam and a sample beam using a dichroic mirror. Whereas the reference light is reflected on a suitably spaced mirror, the sample light undergoes backscattering in different layer depths of a sample to be investigated.
Both reflected and backscattered light are supplied by light guides to the detection unit and as a result of spaced emergence there (cf. two point sources) projected in an at least partly overlapping manner onto a detector plane. This leads to an interference pattern, whose intensity course along the axis linking the light sources enables conclusions to be drawn concerning the light transit times within the sample.
The problem of the invention is to provide a method and a device permitting the contactless temperature measurement of a sample, whose emitted thermal radiation is inadequate for temperature measurement, without there being a thermal contact with a temperature sensor.